GEOLOGY, TAPHONOMY, AND PALEOECOLOGY OF A UNIQUE UPPER CRETACEOUS BONEBED NEAR THE CRETACEOUS-TERTIARY BOUNDARY IN SOUTH DAKOTA

Abstract

This research examines a Konzentrat-Lagerstätte, the Bone Butte site, in the Upper Cretaceous Hell Creek Formation of South Dakota. Field data from a diverse multispecific bonebed in Harding County, South Dakota, including sedimentologic, stratigraphic, taphonomic, floral, and faunal observations, are used to reconstruct the depositional history, depositional environment, and paleoecology of the site, while comparing it to other sites within the Hell Creek Formation. Taphonomic analyses of bone completeness, abrasion, and sorting reveal the presence of autochthonous and allochthonous faunas, and demonstrate the heterogeneity of the Hell Creek dinosaurian fauna. Particularly noteworthy components of the Bone Butte ecology are undescribed insect and ornithurine bird faunas. Also, a rare instance of dinosaur predation is evidenced by theropod tooth marks and an embedded tooth found in two fused proximal hadrosaur vertebrae. The precise temporal resolution of the site, less than 7 years as indicated by sedimentologic, paleobotanical, and vertebrate ontogenetic data, provides the basis of a reconstruction of the local climate and ecology. Finely preserved clay-pebble laminae show evidence of annual rainfall magnitude, and pyrofusain-enriched laminae may indicate a chronology of significant local forest fires. The optimum temperature requirements for plants at Bone Butte, for example the gymnosperm Taxodium, give a rough estimation of annual temperature variation. Thousands of collected vertebrate specimens provide a means of tracking ontogenetic ratios from hatchlings to adulthood in the local stratigraphic column. In some of the dinosaur specimens, the presence of medullary bone allowed estimation of their seasonal nesting habits. Laboratory experiments and examination of modern fluvial analogs augmented the analysis of data from the Bone Butte site. A reconstruction of the Bone Butte river system and site-forming events are presented in this work. In particular, the presence of marine organisms at Bone Butte indicates the river system's close proximity to the paleoshoreline.